The most clinically trialed cells, mesenchymal stromal cells (MSCs), are now known to mainly exert their therapeutic activity through paracrine secretions, which include exosomes. To mitigate potential regulatory concerns on the scalability and reproducibility in the preparations of MSC exosomes, MSC exosomes were produced using a highly characterized MYC-immortalized monoclonal cell line. These cells do not form tumors in athymic nude mice or exhibit anchorage-independent growth, and their exosomes do not carry MYC protein or promote tumor growth. Unlike intra-peritoneal injections, topical applications of MSC exosomes in a mouse model of IMQ-induced psoriasis alleviate interleukin (IL)-17, IL-23 and terminal complement complex, C5b9 in psoriatic skin. When applied on human skin explants, fluorescence from covalently labeled fluorescent MSC exosomes permeated and persisted in the stratum corneum for about 24 hours with negligible exit out of the stratum corneum into the underlying epidermis. As psoriatic stratum corneums are uniquely characterized by activated complements and Munro microabscesses, we postulated that topically applied exosomes permeate the psoriatic stratum corneum to inhibit C5b9 complement complex through CD59, and this inhibition attenuated neutrophil secretion of IL-17. Consistent with this, we demonstrated that assembly of C5b9 on purified human neutrophils induced IL-17 secretion and this induction was abrogated by MSC exosomes, which was in turn abrogated by a neutralizing anti-CD 59 antibody. We thus established the mechanism of action for the alleviation of psoriatic IL-17 by topically applied exosomes.

Topical application of mesenchymal stem/stromal cell (MSC) exosomes have yielded encouraging results in the treatment of psoriasis in pre-clinical studies. The safety of topical applications of MSC exosome in ointment has not yet been determined in human subjects. To assess the safety and tolerability of an MSC exosome ointment, PTD2021P, for topical application in healthy adult volunteers. Ten healthy adult volunteers were enrolled. All subjects received topical treatment with PTD2021P immediately followed by Vesiderm liposome cream ter in die (TID) (thrice a day) on the forearm with a gap of 4 hours between doses for 20 days and underwent another round of screening procedure at end of study (Day 21 + 3 days). Screening procedures included vital signs, blood examinations, photographs and visual assessment of the area of application. All through the treatment period, the subjects completed the daily Subject Diary to capture adverse events, concomitant medications, and time of study product application. One subject was reported with 1 treatment-emergent adverse event (TEAE) of COVID-19 infection during the study. The TEAE was moderate in severity and unlikely related to the study drug. This TEAE was resolved and the subject recovered fully. No subject was reported with clinically significant abnormality for laboratory parameters. As per the visual assessment of the area of application, no subject had dryness, itch, oozing/crusting, redness, scratch marks, skin thickening, sleeplessness, or swelling at the area of application of skin. PTD 2021P was well tolerated for topical application. There were no serious adverse events (SAEs) or TEAEs related to the treatment, and no subject discontinued the study. No clinically significant abnormality was reported for laboratory parameters and vital signs. No abnormality was reported for visual assessment of the area of application of exosome ointment. This ointment could be an alternative therapeutic option for patients who are refractory to current first line therapy.

Mesenchymal stem/stromal cells (MSC) have displayed promising therapeutic potential. Nonetheless, no United States Food and Drug Administration (FDA)-approved MSC product exists due largely to the absence of a reliable potency assay based on the mechanisms of action to ensure consistent efficacy. MSCs are now thought to exert their effects primarily by releasing small extracellular vesicles (sEVs) of 50-200 nm. While non-living MSC-sEV drugs offer distinct advantages over larger, living MSC drugs, elucidating their mechanism of action to develop robust potency assays remains a challenge. A pivotal prelude to elucidating the mechanism of action for MSC-sEVs is how extracellular vesicles (EVs) engage their primary target cells. Given the inherent inefficiencies of processes such as endocytosis, endosomal escape and EV uncoating during cellular internalization, we propose an alternative EV-cell engagement: EMCEV (Extracellular Modulation of Cells by EV). This approach involves extracellular modulation by EV attributes to generate signaling/inhibitory molecules that have the potential to affect many cells within the vicinity, thereby eliciting a more widespread tissue response.

Mesenchymal stem/stromal cell-derived small extracellular vesicles (MSC-sEVs) are promising therapeutic agents. In this study, we investigated how the administration route of MSC-sEVs affects their therapeutic efficacy in a mouse model of bleomycin (BLM)-induced skin scleroderma (SSc). We evaluated the impact of topical (TOP), subcutaneous (SC), and intraperitoneal (IP) administration of MSC-sEVs on dermal fibrosis, collagen density, and thickness. All three routes of administration significantly reduced BLM-induced fibrosis in the skin, as determined by Masson’s Trichrome staining. However, only TOP administration reduced BLM-induced dermal collagen density, with no effect on dermal thickness observed for all administration routes. Moreover, SC, but not TOP or IP administration, increased anti-inflammatory profibrotic CD163⁺ M2 macrophages. These findings indicate that the administration route influences the therapeutic efficacy of MSC-sEVs in alleviating dermal fibrosis, with TOP administration being the most effective, and this efficacy is not mediated by M2 macrophages. Since both TOP and SC administration target the skin, the difference in their efficacy likely stems from variations in MSC-sEV delivery in the skin. Fluorescence-labelled TOP, but not SC MSC-sEVs when applied to skin explant cultures, localized in the stratum corneum. Hence, the superior efficacy of TOP over SC MSC-sEVs could be attributed to this localization. A comparison of the proteomes of stratum corneum and MSC-sEVs revealed the presence of >100 common proteins. Most of these proteins, such as filaggrin, were known to be crucial for maintaining skin barrier function against irritants and toxins, thereby mitigating inflammation-induced fibrosis. Therefore, the superior efficacy of TOP MSC-sEVs over SC and IP MSC-sEVs against SSc is mediated by the delivery of proteins to the stratum corneum to reinforce the skin barrier.

Psoriasis is a chronic, immune-mediated inflammatory disease that affects approximately 2%-3% of the global population, and remains a major dermatologic and psychosocial burden. Despite advances in biologics targeting interleukin 17 (IL-17) and interleukin 23 (IL-23) pathways, effective and accessible treatment options for moderate psoriasis are lacking. Topical therapies and phototherapy are often inadequate, while systemic agents and biologics are limited by toxicity, high cost, and restricted reimbursement criteria, leaving patients with moderate disease without adequate therapeutic options. Mesenchymal stromal cell-derived extracellular vesicles (MSC-EVs) have emerged as a promising acellular therapeutic modality that harnesses the immunomodulatory and regenerative properties of parent MSCs. Unlike systemic biologics, MSC-EVs act locally and non-immunosuppressively. Topically applied MSC-EVs have demonstrated the ability to modulate cutaneous inflammation by attenuating complement activation [via CD59-mediated inhibition of complement terminal component 5b-9 (C5b-9) complex formation], reducing neutrophil infiltration, and subsequently lowering IL-17 and IL-23 expression in psoriatic lesions. Preclinical and early clinical studies suggest that MSC-EVs can restore local immune homeostasis through paracrine extracellular mechanisms, without systemic absorption or adverse effects. MSC-EVs represent a new class of cell-free nanotherapeutics inspired by biologics, capable of localized immunomodulation in psoriasis. By combining biologic-like efficacy with the safety and accessibility of topical therapy, MSC-EVs may bridge the long-standing therapeutic gap in moderate psoriasis. This review discusses current treatment limitations, the mechanistic rationale for MSC-EVs in psoriatic inflammation, and their potential to redefine dermatologic immunotherapy.